1. Field of the Invention
This invention relates to optical information processing and, more particularly, to the optical filtering of RF signals.
2. Discussion of the Prior Art
A spread spectrum radio receiver often is required to operate in an environment where electromagnetic interference (EMI) signals may degrade its performance. Often these EMI signals are relatively stationary in time and frequency while those that are changing do so relatively slowly with respect to the desired signals. For efficient signal reception it is required that the EMI signals be filtered out to the maximum extent possible. It has been the normal practice to employ electronic means to accomplish the filtering function, Electronic filters usually are of the cascade electronic filter type. It has been found, however, that electronic filtering means have a number of disadvantages, particularly where weight and space factors are involved. Weight and volume are proportional to the number of filters required which depend on the number of channels involved. Also, in electronic filters, phase distrortion is additive and nonlinear. Electronic filter elements interact with one another and they themselves are subject to EMI. For these and other reasons, other techniques such as optical filtering of RF signals have increasingly become of interest.
The prior art shows a number of examples of optical systems for detecting and/or removing EMI noise signals from an electrical signal input. An example of an optical matched filter system for recognizing a specific coded pulse is disclosed by J. L. Jernigan in U.S. Pat. No. 3,821,548. A drawback of the disclosed Jernigan system is that it is designed to respond only to a specific predetermined coded pulse and it is not provided with means to automatically adapt the device to filter a variable signal input. However, adaptive or programmable optical systems for filtering electrical signals are known in the prior art as evidenced by W. R. Buchan in U.S. Pat. No. 3,700,902; J. N. Lee, et. al. in a paper "High-Speed Adaptive Filtering and Reconstruction of Broad-Band Signals Using Acoustic-Optic Techniques," 1980, U.S. Army Electronics Research and Development Command, Harry Diamond Laboratories, Adelphi, MD; and T. M. Turpin in a paper "Spectrum Analysis Using Optical Processing", Proceedings Of the IEEE, Vol. 69, No. 1, January, 1981. There are points of similarity between the last three prior art teachings mentioned above and the optical RF filtering system of the present invention but the prior art design approaches militate against the attaining of a practical system that can be implemented for use in adverse environmental conditions such as would be present in an optical RF filtering system designed for use in airborne radar systems and the like.